This invention relates to processes for producing grain-oriented iron for electrical applications, and, in particular to primary recrystallization of relative pure iron.
The design considerations for AC electrical equipment, such as transformers and motors require core materials possessing high permeability, high electrical resistivity, high magnetic saturation, and low core losses. The material must also be sufficiently ductile to permit easy fabrication into thin sheet or strip, and in addition, for transformers must contain a high degree of preferred grain orientation (texture) having an easy direction of magnetization aligned parallel to a specific direction of the sheet. The grain-oriented iron-silicon alloy sheet presently provides a compromise in the above requirements for use in transformers. The addition of silicon to iron has been advantageous for a number of reasons, one of the most important being the suppression of the alpha-gamma transformation which has been obtained with silicon additions of greater than 2.2% silicon (all percent figures herein are weight percentage). For those alloys whose compositions lie outside the gamma loop on the equilibrium diagram, it is possible to perform high temperature annealing which removes impurities and greatly facilitates texture development by secondary grain growth. Silicon additions have also been advantageous for increasing the electrical resistivity, thus lowering eddy current losses, the latter being a component of the total core losses. The presence of silicon, however, lowers the saturation induction from 21,500 Gauss for pure iron to 20,400 for a grain-oriented iron-3.2% silicon alloy. The addition of silicon thus sacrifices saturation induction to obtain the above-mentioned advantages.
While there have been a number of proposals for materials using less than 2.2% silicon, these have generally been processes which (like the higher silicon processes) use secondary recrystallization. Such secondary recrystallization processes generally use a material such as aluminum nitride, manganese sulfide or boron to inhibit primary recrystallization. U.S. Pat. No. 3,573,112 issued to Aspden on Mar. 30, 1971 is a secondary recrystallization process and uses a small (0.00003-0.0005%) but significant amount of sulfur. U.S. Pat. No. 3,636,579, issued to Sakakura et al. on Jan. 25, 1972 uses 0.010-0.065% aluminum in a process which again utilizes secondary recrystallization. U.S. Pat. No. 3,351,501 issued to Aspden on Nov. 7, 1967 illustrates another secondary recrystallization process using sulfur.
A primary recrystallization process is described in U.S. Pat. No. 3,892,605, issued to Thornburg on July 1, 1975. That patent describes a method using an alloyed material containing 0.3-4.0% of one or more alloying agents (e.g., silicon, chromium, or cobalt).